The present invention relates to the measurement of one or more physical parameters using a medical probe.
Medical probes or catheters are used to dispense a fluid inside the body of a patient and/or to measure certain physical parameters inside the body. The parameters may be parameters linked to dispensing fluid, such as the flow-rate or chemical composition, or simply environmental parameters such as pressure, temperature, humidity or pH. The fluid may be a liquid, for example, as when taking intravascular or urinary measurements, or gaseous as when taking pulmonary measurements.
For measuring distal pressure, i.e. at the end of the probe inside the body, the standard method consists in transferring the measurement to a proximal location, i.e. to the outside of the probe in order to measure the pressure with a standard electronic sensor that does not have to be miniaturised. Two main techniques can be used for the interface that enables the pressure to be transmitted: using a liquid mandrel or a gaseous mandrel. These techniques create problems, however, in terms of reliability, user friendliness and accuracy. The direct use of gas as an interface causes damping of the measured signal and is unreliable as the patient""s secretions can block the pressure reading. The liquid-mandrel technique has the advantage of being based on the use of an incompressible interface. It can be used to measure liquids but also to measure gas pressures when a suitable interface is used, as described in U.S. Pat. No. 4,813,431. There is, however, a risk of air bubbles appearing in the column and is therefore potentially dangerous for the patient. In this event steps are required to remove the bubbles. Furthermore, the high density of water in the liquid column creates a difference between the pressure read and the internal pressure that is dependent on the variation in altitude between the two points. International patent 95/22,280 describes a method using laser measurements to estimate this difference but the technique requires a device that is expensive and far from practical to use.
Probes and catheters are used for different applications in the medical sector but on the whole the requirements and technical problems are identical. A particularly significant example is that of mechanical ventilation.
The mechanical ventilation of a patient in intensive care may continue over several weeks, even several months. Specialist s often use xe2x80x9cpressurisedxe2x80x9d ventilation techniques in which the ventilator must reach a level of pressure (controlled pressure) or facilitate patients"" inspiration (assisted pressure). The respirator is guided by the patient""s reactions that it receives via flow-rate- or pressure-sensors. The aggressiveness of the ventilation techniques is preferably limited in order to prevent the patients"" state of health from worsening, accelerate their recovery and gradually encourage patients to breath on their own again. When patients still have or have regained their breathing reflexes they trigger gas to be dispensed from the respirator. It is therefore essential for sensors, particularly pressure sensors, to be sensitive and reliable in order for mechanical ventilation to be suited to patients"" actual requirements.
At present the pressure sensors are located on the external circuit that connects the patient to the mechanical respirator. The signals emitted by the sensors do not reflect the actual conditions in vivo due to the difference caused by the connecting parts, particularly the intubation probe. This phenomenon is particularly noticeable in the phase during which the probe is removed from the patient when the flow rates are immediately increased. The gaseous mandrel technique is marketed by all the probe producers but it is too unreliable for use in controlling the respirator directly.
Direct, reliable measurements of the pressure inside the respiratory tracts result in significant progress for the safety of patients and open the way to developing ventilation techniques with improved performance characteristics that will reduce the average stay in intensive care units and will therefore have a positive effect on hospital costs. The usefulness of such techniques is increased with the new, high frequency ventilation methods used on adults and particularly young children, for whom no accurate routine monitoring means are currently available.
Another example concerns urodynamics. Certain urology examinations require liquid to be injected into the patient""s bladder and the changes in pressure to be monitored. At present catheters fitted with instruments that include electronic-pressure sensors, which are also very fragile, are expensive. They are therefore mainly reserved for research applications. Consequently, measuring the physiological liquid-supply pressure is preferred. The technique is, however, difficult to implement and unreliable due to the problems caused by removing bubbles and the pressure difference resulting from the height of the column of injected liquid.
Another example concerns the probes used in the cardiovascular sector. The arterial pressure is measured outside the body at the proximal end of a catheter filled with pressurised physiological liquid. This technique requires the use of a pocket of pressurised serum and a control valve that take up space around the patient unnecessarily. This application illustrates the use of a catheter containing a pressure sensor but that does not dispense a fluid.
A technique is therefore required, particularly for measuring pressure, that integrates the electronic sensor directly at the end of the probe or catheter in the actual place where the measurement is necessary. The requirement extends to measuring several parameters at the end of a catheter.
For reasons of hygiene the probes are preferably used once only. Ideally they should therefore be produced for a moderate cost price. They are produced using extrusion techniques that enable the cost objectives to be reached, even for medium-sized production runs. The techniques are restricting in terms of materials and shapes as they require the probe to be symmetrical around the longitudinal axis. Techniques are known that enable probes to be produced with an end in a different material from that of the probe body (see for example U.S. Pat. No. 3,890,976 and international patent 94/00174). Techniques are known for measuring pressure at the end of a probe using an electronic sensor that is implanted directly in the probe and that is connected to the outside of the patient""s body by leads. They are used to measure intratracheal pressure (see international patent 94/22518) or arterial pressure (see international patent 97/17888). Multiplexing techniques have been described to enable a single pair of leads to be used (see U.S. Pat. No. 4,432,372). These techniques, however, lead to significant increase in cost that arises from the sensor being fastened inside the probe and the electrical connections that are not directly compatible with the extrusion techniques. Their use is therefore limited to only a few applications.
In general, and particularly concerning pressure, the production of microsensors is dependent on the microtechnology that is currently being developed through the progress made in the microelectronics industry. The techniques that are being developed today enable mechanical functions to be integrated into electronic components and miniature electronic sensors to be designed. Compared to standard mechanical sensors the new sensors are more sensitive, more reliable and multi-purpose because they are capable of being connected to a signal processing unit.
For pressure sensors in particular, various techniques for measuring the distortion of a membrane have been proposed for use in these systems. Piezoresistive techniques are used to measure the distortion of a piezoresistive part placed on the surface of a membrane. The pressure is determined by measuring variations in resistance. Today this technique is limited by the minimum size of the pressure sensor required and the consumption of the detection system. The use of an optical system (see U.S. Pat. No. 5,546,939) for measuring distortion simplifies the problems of electrical connections but it is difficult to use for measuring several parameters and does not enable in situ signal processing to be integrated. Capacitance techniques are currently the most promising in terms of space requirements and power consumption. They require, however, a capacitance variation electronic processing system to be available in the immediate vicinity.
Microsensors are usually connected to data processing devices by connecting wires or optical fibres. U.S. Pat. No. 4,127,110 describes a version of a wireless microsensor that is used with capacitance sensors to measure intracranial pressure. The capacitance of the sensor is used as the basis of an L-C circuit the resonance frequency of which measures the variations in pressure. This technique is, however, limited to obtaining a single parameter and the transmission of the measurement is necessarily analogue, which limits its accuracy. U.S. Pat. No. 4,556,063 proposes extending this technique to measuring several parameters in implanted circuits that have a battery power supply. This technique is currently used to programme heart pacemakers. Following the progress made in miniaturisation it is possible to apply remote power supply and remote transmission techniques using miniaturised circuits. These techniques have the advantage of providing greater flexibility in terms of processing. Their application to systems that only use passive measuring circuits is known for implantable systems (for example see U.S. Pat. No. 5,704,352).
The main problem when using microsystem-based components in medical probes is the cost of integrating them.
In order to provide a solution to the problem described above, it is proposed that the mechanical part of the probe (the rod) that is produced using a standard, inexpensive technique without incorporating any electronic components or electrical connectors, and then adding an electronic measurement unit that comprises one or more sensors, a local miniaturised electronic circuit that enables the measurement signal to be processed and a component that transmits the measured values and receives a remote power supply signal.
The aim of the invention is therefore to provide a system for measuring at least one physical parameter in a place in a patient""s body to which a medical probe has access, comprising a medical probe equipped with a sensor of the said parameter and means to emit an electronic signal representing the said parameter that is received by the sensor, to a data processing device outside the patient""s body, characterised in that:
said probe consists of a rod comprising means to fasten it to an electronic measuring unit,
the sensor of said parameter is included in the electronic measurement unit that also includes other parts consisting of electronic means associated with the sensor to provide a measurement signal, means for remote transmission of the measurement signal, power supply means for the electric means associated with the sensor and remote transmission means, the measurement unit also comprising additional fastening means besides those of the rod of the probe,
the means for emitting an electronic signal representing said parameter to the data processing device are receiver means positioned such that they are capable of receiving the measurement signal emitted by the remote transmission means,
the remote transmission means comprise a coil-shaped antenna,
the power supply means comprise a circuit capable of being charged by a remote power supply using said coil.
The fastening means of the rod may comprise a housing that enables the measurement unit to be inserted. If the rod is hollow said housing may be provided in the internal wall of the rod. The measurement unit may be ring shaped, said housing constituting an annular throat in the rod. If the measurement unit is a closed ring the rod may be produced in a material that is sufficiently elastic to enable the measurement unit to be inserted into the housing due to distortion of the rod. The rod can also comprise a narrowing against which the measurement unit comes to stop. In this event the rod can also comprise an anchoring system that retains the measurement unit in the housing. The measurement unit can be an open ring that clips into place in said housing.
The fastening means between the measurement unit and the rod may comprise contact surfaces between the measurement unit and the rod. The fastening means between the measurement unit and the rod may comprise an adhesive substance or be based on moulding techniques.
The receiver means may be positioned on the end of the probe that is outside the patient""s body. They can also be means that may be placed on the patient""s body.
The remote transmission and reception means can be radio-frequency, infrared or ultrasound communication means.
Advantageously, the measurement unit comprises a connection plane provided with conductor paths that ensure electrical connection between the various components of the measurement unit. The connection plane may consist of a flexible substrate that is coiled into a tube shape and imbedded in a moulding substance. The components of the measurement unit can have electrical contacts welded onto certain conductor paths selected from said conductor paths. They can be components inserted into the measurement unit, said insertion creating electrical contact between these components and the conductor paths selected from among said conductor paths.
As the remote transmission and reception means are radio-frequency communication means, the measurement unit can comprise an antenna produced by metallisation being deposited on the flexible substrate. The antenna can be an added part that is connected to the connection plane and imbedded in the moulding substance. It can also be positioned around the measurement unit.
For mechanical ventilation purposes the medical probe can be an intubation probe capable of dispensing gas. It can also be a urinary probe capable of taking urodynamic measurements or a catheter capable of taking intravascular pressure.